Great question - and there's no real "correct" answer - as we all started at different times and have our own opinions.

I think DMDs are more complex and costly to own and repair. That's because I've grown up on EMs and SS. I'm very comfortable around EMs, and I've never "given up" on a repair. Most sellers will tell you they are "Nothing but trouble", "too hard to fix" or "there are no spare parts for them" because they don't have qualified (experienced) staff to maintain them. Go to Pinball Resource's site and you will see thousands of parts for Gottliebs. Unfortunately, EMs don't have the appeal to the younger generations, as there are no "bells and whistles" - But EMs have something DMDs don't have - character.. Well DMDs have not been around as long as EMs... 

Solid States are my favorite and I'd say that they have made a huge comeback in the last 2 years. Some of the best machines were made in this era, and my all time "top 10" would be mostly SS machines from 1978 - 1982. Parts are available and they are easy to fix..

DMDs are still great fun and I look forward to restoring my CV and one day I might add a pinball magic or Scared Stiff to my collection.

I'm comfortable with maintaining all three formats. If you find an EM or SS that you really want to add to your collection, don't be too concerned about reliability, ease of repair and troubleshooting of EMs or SS - Stick to Gottlieb, Williams or Bally and you can't go wrong !

Finally - It also depends on your ambition to learn about older games. There's plenty or resources out there and sites like these that have members only to happy to assist !

Footnote - I've pulled EMs out storage and powered them on for the first time in 20 years.. They worked !!